Method of operating digesters and apparatus therefor



Dec. 27,- 1938. E, M, FRANK L AL 2,141,384

METHOD OI OPERATING DIGESTERS AND APPARATUS THEREFOR Filed April 10, 1935 INVENTOR ATTORNEY Patented 27, 1938 UNITED STATES PATENT OFFlCE METHOD OF OPERATING DIGESTERS AND APPARATUS THEREFOR Application April 10, 1935, Serial No. 15,510

2 Claims.

Our present invention relates to improvements in the manufacture of pulp by the sulfite process and more particularly to a method and apparatus providing an improved control of temperature 6 conditions within the digester. In the sulfite method the raw eellulosic material is placed in a digester together with the treating liquor containing free sulphurous acid and the cookin carried out by the admission of steam under pressure. As the art has progressed and the size of the digesters increased, greater difficulty has been experienced in maintaining accurate and uniform control of temperature conditions within the digester. By way of eliminating this difllculty, it has been proposedto install devices for mechanically circulating the liquor within the digester but such devices are cumbersome and have not been altogether successful. One way of improving these conditions has been to 2 charge the digesters with hot acid cooking liquor, say at temperatures from 70 to 80 degrees C., such hot charge facilitating the better control of temperature conditions within the digester. To obtain such controlled conditions, the use of steam flow meters has been proposed by means of which the operator could vary the steam in accordance with a predetermined schedule which was calculated to give the desired result. The dimculty has, however, been experienced that such meters do not measure the steam flow with sufficient accuracy to give a final temperature within the limits of precision desired. This is due to the fact that steam flow meters are normally only accurate to about 2%% and in some cases the error rises to about 5%. With a rise in temperature of say '70 degrees C., an error of this magnitude would give a final temperature in the digester 3 or 4 degrees above or below that desired, such an error being serious and impair- I ing the quality of the product.

As a result of considerable experimentation, we have found that it is possible to obtain a high .degree of uniformity of heating of the charge by feeding thereto steam under constant flow while relieving the digester under substantially constant pressure. This constant fiow of steam may be had in a number of ways. The best way we have found is to provide. first. a source of steam under constant pressure and then to have within the steam pipe to the digester a plate having a fixed orifice. Since the relief pressure is constant, the pressure on the downstream side of the orifice plate will also be constant. and since the pressure on the upstream side is also i maintained constant, an accurately uniform flow of steam to the digester results. The size of plate may be determined by calculation knowing the steam requirements for a given size of digester, or by the use of a steam flow meter. Should, after trial, it be necessary to vary the 5 size of the orifice slightly, this may be done by substituting a new plate having the desired size of orifice.

It will be understood that the means by which the orifice is provided is quite immaterial so long 10 as such orifice remains invariable. Thus, it is possible to use valves of various types provided that once the correct size of orifice is obtained it I is not thereafter varied. However, because of the simplicity and accuracy of an orifice plate and because in the case of a valve the same setting is dimcult to reproduce, an orifice plate is therefore preferred although we do not limit the invention to its use. In contrast to the error involved in the use of a steam flow meter of from 2.5 to 5%, the use of an orifice plate of the proper size gives an error less than 1%.

One corollary of our method of operation is that because of the strictly uniform feed of steam a straight line time-temperature curve with reference to the digester charge results, which curve by the choice of the orifice can within limits be made as flat or as steep as desired. As an illustrative example the digester may be charged with acid liquor at from to so degrees C. and steam added at a uniform flow so as-to arrive at an end temperature of from to degrees in from five to seven hours. ,The steam is thereupon shut off and the charge held for from two to three hours and 35 the digester blown. While it is preferred to charge the digester hot, this is not absolutely essential although naturally a longer time will be required for the cooking operation if the digester is charged with cold liquor.

Our invention will be further understood by reference to the following detailed description taken with the annexed drawing in which the figure illustrates a steam header and one of a plurality of digesters served thereby, such apparatus containing a preferred embodiment of the invention.

In the drawing, steam from source I passes through a regulator valve 2 operating to maintain a constant pressure of steam within theheader 3. This valve arrangement is of known type andneed not be referred to in detail other than to say that the valve 2 is controlled by a diaphragm l responsive to pressure within the header 8 which is conveyed to said diaphragm through pipe 5, the diaphragm 4 being connected to the valve 2 by means of a link 6, all in a manner well understood in the art. From the header 3, the steam under constant pressure passes through a fixed orifice I into a line 8 entering the digester 9' at the bottom thereof. The digester 9 is provided with a constant pressure relief valve In similar in principle to valve 2, the same being operated by pressure within the digester which is conveyed to the diaphragm through pipe ll.

Previous to admitting steam through the pipe 8, the digester is charged with raw cellulosic material, as for example pulp wood chips, through the top 12 and then with preferably hot acid liquor. The top I2 is then closed and steam is admitted by fully opening valve l3 which thereafter is not disturbed during the cook. The cock is continued until the desired temperature (as revealed by a suitable temperature indicating device, not shown) is reached whereupon the steam supply is shut oif, the digester held at substantially this temperature for a desired period and then blown in the usual manner by opening the valve l5.

From the foregoing it will be apparent that the invention results not only in a more accurate operation but in great simplification of apparatus with the elimination of costly and complicated control devices. It will be understood that various changes in the apparatus and in its manner of operation will occur to those skilled in the art without, however, departing from ,the spirit of our invention or the scope of the appended claims.

We claim:

1. In a cooking system for manufacture of paper pulp, a digester", a means to relieve same at constant pressure, a pipe supplying the steam to said digester, and means for maintaining a uniform flow of steam through said pipe, said means comprising an orifice plate and means for supplying steam at constant pressure on the upstream side of said plate.

2. In the method of pulp manufacture consisting in cooking cellulose material with acid liquor in a digester and relieving the digester at constant pressure in excess of a predetermined maximum, the improvement which consists in bringing the digester up to said maximum pressure solely by applying steam under constant pressure on the upstream side of a fixed orifice chosen with respect to the steam requirements of said digester, said requirements being constant during the period of steaming, conducting the steam passed by said orifice to the digester and maintaining said constant steam pressure against said orifice and the steam fiow resulting therefrom throughout the entire steaming period whereby no further regulation of the steam flow is had during the heating period.

EDWARD M. FRANKEL. EDWARD H. BARRY. 

